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As a note, everything written in here is as Ouma knows things to be. By the end of the game we learn that everything was fabricated, including their memories; however, it's those memories that are used here for his personality.

While Ouma is considered to be the embodiment of a "lie", we are never given a clear cut reason for this. For the most part, the game only alludes to Ouma's past through the viewing of his motive video and the lies he tells. From his motive video we know that outside of the killing game, Ouma has 9 people that he is close to: his organisation, DICE. It's hinted that these teens either lost or didn't have a family, and that's why they banded together. When Tojo, the Ultimate Maid, takes care of the 16 of them trapped inside the killing game he asks if she could be his mom. That seems to hint towards him actually wanting that kind of relationship in his life, or that he simply hasn't had the luxury of it. Tojo does seem to treat him that way too, such as when she scolded him outside the gym for almost an hour for "being up to no good". DICE get by in the world by doing petty crimes, but also do pranks on people for the hell of it. It's hinted that part of Ouma's distrust could also be that someone hurt him emotionally. That's why he lies so much now to avoid actually letting anyone get close to him.

In the whole game there are only a select few who seem to truly garner his interest. As he loses Akamatsu and Amami early on (Akamatsu as the "first" protagonist and Amami as someone the anthologies hint as him looking up to in a "big brother" kind of light), it's hard to say if his relationship with them would go the same way as it does with Saihara. Saihara doesn't seem all that special in chapter one, it's only after Akamatsu's death and Ouma sees how he grows during the trial that he actually starts to take notice of him. By chapter four, mind you, Ouma is actively trying to gain his attention and approaches Saihara to ask him to work together during the investigation. He believes that together they'll be able to solve everything, the only problem is that by that point he's already ruined it for himself - Saihara doesn't trust him. Quite rightfully, honestly. After manipulating Gokuharu into gathering everyone to watch the motive videos together and constantly lying, he hasn't exactly given Saihara a reason to trust him. The role he's acting out is a lie but he seems to expect Saihara to see through it because of his talent as Ultimate Detective. On top of that, if you partake in his free time events, he plays a set of games with him that are meant to act as a bet on Saihara's life, yet he purposely lets himself lose and says it doesn't matter because he's stolen Saihara's "heart" which is also like winning the game.

His relationship towards Saihara definitely isn't healthy, though that may be because of the situation he's put himself in by blocking out everyone else. When he asks Saihara to work with him and is refused, he has a moment to himself where he says that whenever he has someone who interests him, he'll even strangle them if he has to - this should not be taken literally. What he really means by this is that he's willing to be overbearing to get someone's attention. It's more like "suffocating" someone by being there constantly and not giving them a break. In this way, he's also more like a child than a teenager too, as children have no concept of personal space. Despite how he can take things too far, however, there is a moment where he seems to be genuinely happy. When he accidentally cuts himself during a knife game with Saihara, the other boy bandages it for him. He seems really pleased that he managed to make Saihara worry about him as he would for any of the others.

(I'd also like to note that there are variations on this given the differences between the Japanese and English translations. Some people have translated it as "would let them strangle me" and that also has issues. This is the interpretation I've taken from it, given what his character is like and how he tries to push himself between Saihara and Momota after this as well.)

The fact that he behaves quite childishly also fits with his past as well. He likes playing games and gets Akamatsu and Tojo to play tag with him, says inappropriate things at the wrong times like he hasn't been taught to keep his mouth shut and even exaggerates his reactions to make them seem even more like a farce for attention. On top of that, he insults the others, calling them dumb, bitch, whore etc to keep them at a distance, but it is also partly due to the fact that he does see himself as a lot more intelligent than some of them. Momota, Gokuhara and Iruma especially get those kinds of insults during the trials as he has to explain what he means so they can understand as well. This is something that seems to frustrate him.

Throughout the game, the others seem to underestimate his intelligence and perceptiveness, especially in the earlier chapters. Ouma warned Akamatsu that she was inviting danger with her optimistic attitude. Despite his warning, she didn't listen and was targeted by the mastermind. The fact that he forebodes this shows that he's able to read other people well, and also what kind of response their actions may cause. Part of the reason for this is because he spends so much time studying other people. The whiteboard in his room that we see in chapter 6 is an example of this as he's made notes about the other students.

It's also very likely that after the events of chapter four, Ouma felt guilt for his involvement in Iruma and Gokuhara's deaths. He wasn't the one who set the ball rolling, that was Iruma, but it was still him who made the plans that ended with Gokuhara killing Iruma. When Gokuhara is revealed as the culprit and everyone votes, Ouma asks to be executed with him. He later says that wanting that was a lie, but that seems rather doubtful due to his actions throughout the whole game and what we know of his dislike of murder. He likely felt trapped after realising Iruma was trying to kill him, and did the only thing he could do to prevent it. If Iruma had succeeded, not only he would have died, but it's likely she could have tricked everyone into thinking someone else was the culprit and everyone but Iruma would have died with him. If anything, those actions likely made him hate himself, but it was also the step that finally made him play the role of the villain as he does.

Ouma later pretends to be the mastermind of the game to lure the real mastermind out. The fact that he's willing to villainise himself for the other's sakes shows that he can and does care for other people. His willingness to go as far as he does (getting himself killed, getting Momota to work with him in such a twisted plan) is something that most people would be too afraid to go through with. If Ouma was really as cruel as he made himself out to be, he would have just taken the antidote to the poison for himself after Harukawa shot him, but instead he gives it to Momota and tells him to help him so he can save Harukawa. Ouma's plan was meant to end the killing game for good. The reason for that was because despite lying to the others and himself that he actually liked the killing game, he actually hated it. Killing is actually against everything Ouma stands for. His group, DICE, even has a rule stating that murder is forbidden, so why would he say he is enjoying the game? That was a ruse to keep the mastermind off his back and give him the freedom to investigate as necessary.

He can be quite helpful in his own way. During trials he leads the others in the right direction or opens up topics to talk about when they're stumped. This is because he isn't lying when he says he doesn't want to die, and in actuality he doesn't want any of them to die. He helps on a personal level as well, including getting Yumeno to cry after Angie and Tenko's deaths so she can move on. He doesn't do it in a clear cut way so it isn't obvious to the other students, but it's definitely meant to help her rather than make her feel worse.

Ouma can use special tools to pick locks. In the game we see him pick a cylindrical lock, but given that he can identify locks from their appearance, it's safe to say he's likely able to pick all kinds of locks.

He's a compulsive liar to the point where everything he says sounds like a lie. It's hard to tell when he's telling the truth.

Ouma is able to work out crimes easily, especially after investigating on his own. This shows that he's perceptive and is also intelligent.

The game also alludes to the fact that he might be able to speak other languages, which fits with the fact that DICE apparently travelled the world.